Many varients of the FakeAlert around, which disable McAfee, and other apps.
Boot to a boot cd and manually remove the threat(s). By this i mean go into safe mode (if you can).
Malwarebytes, great tool:
http://www.malwarebytes.org/
Personally I would remove mcafee, install NOD32 Antivirus/Antispyware, update it, scan, then run an online scan with kaspersky (to verifiy), then run Malwarebytes [good for polymorphic malware], and lastly a full scan (not default quick scan) with Defender- also check the boxs for notification of software that creates system changes under Tools>Options: (this will notify you in the event malware tries to make changes (i.e., new run executables), and allow you to prevent it from running.
Be sure to have updated DATS, as the fakealert varients update themselves each hour or more!
What O.S. do you have?
Let us know how you go.